Sir Gawain and the Green Knight is a 14th century poem by an anonymous poet. It is a chivalric romance set at King Arthur's court. It tells the story of Sir Gawain, Arthur's nephew, who takes up a challenge from the mysterious Green Knight. It revolves around honour, honesty, and chivalric themes like hunting, seduction and promises.
It is written in Middle English, so much more easily understood than a work like Beowulf. However, to the untrained eye, this English still looks rather foreign. Have a look at the four opening lines for an idea.
"Siþen þe sege and þe assaut wats sesed at Troye,
þe borgh brittened and brent to brondez and askez,
þe tulk þat þe trammes of tresoun þer wroght,
Watz tried for his tricherie, þe trewest on erthe."
The poet uses an interesting combination of alliteration, which was popular in Anglo-Saxon times, as well as end rhyme, made popular in and imported from medieval France. It uses the bob and wheel technique to combine these two. This, and the extent of his descriptive vocabulary, marks him as a very skilful poet.
Check out the video below for an idea of how Middle English sounds like.